Due to the excellent characteristics of field-effect transistor and its high absorption coefficient in the visible region, pentacene has been widely used in phototransistors. The channel length of the vertical transistor could be designed to be very short (on the order of nanometers). In this way, the device performances and its working frequency can be effectively improved, and the energy consumption can be reduced simultaneously. In this paper, we fabricate a kind of low-voltage pentacene photodetector ITO(S)/Pentacene/Al(G)/Pentacene/Au(D), based on the vertical transistor configuration. The threshold voltage and “on/off” current ratio are -0.9 V and 104 at a low working-voltage of -3 V, respectively. The pentacene photodetector ITO/Pentacene(80 nm)/Al(15 nm)/Pentacene (80 nm)/Au exhibits a good p-type transistor behavior and low-voltage-controlling performance. The photosensitivity and responsivity vary with incident monochromatic light from 350 nm to 750 nm, and the photosensitivity peak of 308 is obtained at 350 nm with a responsivity of 219 mA·W-1, which is even higher than that of the standard Si-based photodetector under 350 nm incident light. Therefore, this work provides an easy way to fabricate a high sensitivity all-organic photodetector working at low voltages.